PROJECT SUMMARY Barth syndrome (BTHS) is a rare, frequently fatal, mitochondrial disease caused by recessive loss-of-function mutations in the gene TAZ, which encodes tafazzin. Tafazzin is a nuclear-encoded transferase that is trafficked to the inner mitochondrial membrane where it remodels monolysocardiolipin (MLCL) to mature cardiolipin (CL). A critical phospholipid, CL is involved in maintenance of mitochondrial membrane fluidity, osmotic stability, and efficient respiratory chain function. In BTHS patients, improper MLCL:CL ratios result in decreased mitochondrial energy production and cardioskeletal myopathy. Although deficient CL remodeling exists in all BTHS patients, there are considerable differences in disease progression and clinical presentation and the basis for these differences remains obscure. These differential clinical outcomes combined with the high variability in the location and severity of TAZ mutations suggest that tafazzin possesses uncharacterized functions that may involve interactions with unidentified proteins to influence BTHS and common heart failure presentation as well. In order to define these novel mechanisms, we propose to 1) evaluate overall protein expression profiles as well as mitochondrial morphology, turnover, and function in human BTHS patient induced pluripotent stem cells (iPSCs) differentiated into cardiomyocytes (CMs) and skeletal myotubes (SkMs) using both standard culture systems and biomimetic microenvironments, 2) compare iPSC-CM and -SkM phenotypes to human clinical skeletal muscle and cardiac functional, metabolic and energetic indices, and 3) design human native and mutant tafazzin constructs with missense mutations to evaluate alterations in tafazzin protein localization and complex binding patterns. TAZ gene replacement and subsequent analyses will confirm that the observed effects are caused by tafazzin deficiencies. The in vitro work will be complemented by in vivo BTHS mouse model analyses and human data from an ongoing clinical study. In combination, these models will provide valuable platforms for defining disease mechanisms and testing pre-clinical gene therapy. We hypothesize that evaluation of functional abnormalities, identification of tafazzin protein complex binding partners and their impact on expression profiles in mitochondria from BTHS iPSCs differentiated into CMs and SkMs representing a variety of distinct TAZ mutations and a BTHS mouse model will reveal novel roles for tafazzin. As decreased TAZ expression has been associated with a variety of non-BTHS heart disease states and CL abnormalities have been detected in a range of disorders, mechanistic knowledge gained from this study has great potential to improve our general understanding of and provide novel therapeutic targets for a wide range of health concerns that stretch well beyond BTHS.